1. Field of the Invention
This invention relates to a method for processing a layer of the silicon-based material applicable to fine processing, such as in a semiconductor process. More particularly, it relates to a method for post-processing for a dry etching process employing a silicon compound as a sidewall protective film.
2. Description of the Related Art
Recently, in keeping with the increasingly high integration degree of an integrated circuit, such as VLSI or ULSI, the design rule in semiconductor devices has also been refined. In the field of dry etching, there has been raised a strong demand for simultaneous attainment of high selectivity, practical etch rate, low pollution and low damage at as satisfactory a level as possible. However, since these requirements need to be compromised among one another, the present-day practice is to perform etching by suitably adjusting these requirements within a practicable acceptable level.
Heretofore, chlorofluorocarbon gases (CFC gases) as exemplified by FLON 113 (C.sub.2 Cl.sub.3 F.sub.3) have been widely employed for etching a silicon-based material, such as single-crystal silicon, polysilicon, high melting metal silicide or polycide. Recently, however, stringent regulations are put on the use of CFC gases, to say nothing of gases designated as specified FLON, such as the above-mentioned FLON 113. Thus a variety of alternative gases have been proposed.
Many of these alternative gases employ halogen-based chemical species other than fluorine-based chemical species, that is, chlorine-based or bromine-based chemical species, as main etchants. It is specifically intended to realize shape anisotropy by an ion-assist mechanism by Cl.sup.+ or Br.sup.+, instead of by fluorine radicals F* liable to spontaneous chemical reaction with the layer of the silicon-based material, and to realize high selectivity for a gate insulating film (SiO.sub.x), a material for an underlying layer in case of performing gate electrode processing.
Recently, there has also been proposed a method of performing etching using oxygen-based chemical species in co-existence with the above etchants, such as Cl- or Br-based etchants, and realizing sidewall protection using reaction products such as SiO.sub.x based materials.
In the Extended Abstract to the 39th Lecture Meeting of the Association of Applied Physics (Spring Meeting of 1992), lecture number 28p-NC-4, page 504, there is disclosed a technique of coating a resist mask with the SiO.sub.2 based material in the Si etching employing the HBr/O.sub.2 mixed gas for inhibiting the recession of the resist mask for achieving high shape anisotropy.
The formation of, for example, a gate electrode by etching under co-existence of the Cl- and Br-based etchants and the oxygen-based chemical species as described above, will now be explained in detail.
Referring to FIG. 1, a resist mask 7, patterned to a pre-set shape, is formed on a wafer comprised of a gate insulating film 2, a polycide film 5 made up of a polysilicon layer 3 and a high melting metal silicide layer 4, and an antireflection film 6, stacked in this order on a silicon substrate 1.
Then, using the resist mask 7 as a mask, an HBr/O.sub.2 mixed gas is subjected to discharge dissociation in order to carry out reactive ion etching (RIE) for etching the polycide layer 5. By such RIE, an area not covered by the resist mask 7 is removed, while the resist mask 7 is protected by a sidewall protective film 8, as shown in FIG. 2.
The sidewall protective film 8 prevents the resist mask 7 from being receded by etching, and thus contributes to attainment of the shape anisotropy. The sidewall protective film 8, composed mainly of an SiO.sub.x -based product yielded on direct oxidation of Si atoms expelled from the polycide film 5 or on oxidation of the etching reaction product SiBr.sub.x, also contains carbonaceous reaction products yielded by forward sputtering of the resist mask 7.
After etching the polycide film 5 as discussed above, it is necessary to carry out removal of the resist mask 7 and the sidewall protection film 8 by way of post-processing. Such post-processing is usually carried out by processing with a dilute hydrofluoric acid solution followed by plasma processing using an oxygen-based gas.
It is specifically intended by this process to remove the sidewall protection film 8 of the SiO.sub.x -based products by dissolution with the dilute hydrofluoric acid solution and subsequently remove the resist mask 7 of the organic material by plasma processing with the oxygen-based gas. However, even if processing with the dilute hydrofluoric acid solution and the plasma processing with the oxygen-based gas are carried out in this order, the sidewall protection film 8 is actually left as shown in FIG. 3, such that sufficient removal can hardly be achieved.
If the wafer is sent to the next process, such as the process of forming an interlayer insulating film, with the sidewall protective film thus being left, the sidewall protective film tends to be scattered to produce dust or deteriorate the coverage by the interlayer insulating film.